Fabrication of semiconductor devices often requires use of capping or encapsulating surface layers for surface protection and pattern delineation. Such surface layers are useful during the fabrication as well as on completed devices.
U.S. Pat. No. 4,731,293 issued on Mar. 15, 1988 to D. T. Ekholm et al., which is incorporated herein by reference, describes the use of phosphosilicate glass (PSG) in the production of semiconductor devices including devices comprising III-V and II-VI compound semiconductors, such as GaAs and InP and compound semiconductors lattice matched to GaAs and InP. The PSG comprises at least 60 mole per cent silica and up to 25 mole percent P.sub.2 O.sub.5 and may contain up to ten mole per cent of other components such as B.sub.2 O.sub.3, rare earth oxides (R.sub.2 O.sub.3), etc., with less than one percent being preferred. The PSG is deposited on the semiconductor by a particle-beam deposition including e-beam deposition and sputtering. E-beam deposition is preferred because the surface of the semiconductor on which the phosphosilicate glass is being deposited is less likely to be damaged and involve contamination of the semiconductor surface. To be useful for particle beam deposition the glass should be of homogeneous composition, amorphous and transparent.
Prior art procedure for the preparation of PSG targets for e-beam deposition, as described in the above mentioned U.S. Pat. No. 4,731,293, involves dry milling of ingredients in a dry, inert atmosphere followed by heat treatments to react the component oxides, make the glass uniform and remove moisture and/or bubbles. The heat treatments include heating a milled mixture of ingredients in an oxygen containing atmosphere at a temperature between 1300.degree. and 1500.degree. C. for five days, and heating the resultant glass at 1850.degree..+-.50.degree. C. for 5 days in an inert, e.g. argon, atmosphere.
This procedure is very expensive and energy and time consuming. Furthermore, during the high temperature firing some of the phosphorus present in the initial ingredients is lost. For example, a starting material containing 12 mole percent P.sub.2 O.sub.5 may yield a final glass with about 10 mole percent P.sub.2 O.sub.5. This leads to difficulties in obtaining PSG with lower P.sub.2 O.sub.5 content, such as 1-15 mole percent P.sub.2 O.sub.5. Therefore, it is desirable to produce the PSG targets by a process which would overcome disadvantages of prolonged heating at high temperatures.